One-piece cylindrical bits of the above-mentioned type, used primarily to machine wood or plastic, have been known for decades in various embodiments, and are, for example, shown "Drilling and Milling Tools," 1990, FAMAG-Werkzeugfabrik Friedr. Aug. Muehlhoff, Remscheid.
Depending upon the length of the circumferential cutters, a distinction is made between Forstner bits and knothole cutters, which have especially long circumferential cutters, and so-called multispur machine bits in which the circumferential cutters, generally termed precutters, each extend over only about one-tenth to one-twelfth of the circumference of the drill bit. Spaces and channels to carry away the chips that are produced must be provided for both the main cutters and for the circumferential cutters. Proposed cylindrical bits have proven to be very good in service, but chip removal poses problems when deeper holes must be made with the holes being impossible to make in one work step, but only with intermediate removal of the bit from the hole, since known one-piece cylindrical bits have relatively polygonal spaces machined in the drilling head, in which chips can accumulate.
In, for example, U.S. Pat. No. 4,090,807, a cylindrical bit made in several parts is provided with a central piece threadably inserted into an end of the shank, into which piece a centering tip is in turn threadably inserted in turn. On the sloping lateral surfaces of the centering tip, two plates bearing the main cutters are welded, with each plate having, at its rear, an approximately semicircular radially projecting segment welded in place, on which precutters are formed. The radially projecting segments are fashioned as relatively flat disks and provided with radial corner recesses delimited in each case by the plate of the other main cutter to form a chip removal opening. The bottom of the chip removal opening, located radially inward, is rounded in an arcuate shape. The flat design of the segments does reduce problems associated with chip removal, but this is accomplished at the expense of the stability of the cylindrical bit. The main cutters and precutters project roughly at right angels beyond the ends of the segments and, in turn, form polygonal chambers in which chips can accumulate. Moreover, this multipart cylindrical bit is very expensive the manufacture.
In, for example, U.S. Pat. No. 3,945,753, another cylindrical bit is threadably assembled from five parts, in which beam-shaped main cutters are provided, on whose radially external corners small axially projecting precutters are milled. Both the main cutters and the precutters project only slightly beyond a slightly roof-shaped end of the drilling head, so that very narrow spaces are produced in which chips can readily accumulate. This known drill is not suitable for drilling deep holes in one work step and is also very expensive to manufacture.